In optical disk apparatuses, it is necessary to always keep a light beam in a preferable state of convergence with respect to the recording face of an optical disk when information is recorded or reproduced. For this reason, a controlling operation called focus (or focusing) servo is carried out.
FIG. 39 shows one example of the focusing control device that is used in a conventional optical disk apparatus. In order to record and reproduce information, an optical pickup 91 converges a light beam 93 onto a recording face 92a of an optical disk 92 through a lens 94. Further, the optical pickup 91 obtains a signal derived from the reflected light from the optical disk 92 by the use of an optical system, not shown, that is installed therein. The signal is sent to an error-signal generation circuit 95. The error-signal generation circuit 95 generates a focus error signal FES, a tracking error signal TES, and other signals from the signal. Here, the focus error signal FES indicates a deviation between a converging position of the light beam 93a and the recording face 92a in the perpendicular direction, and the tracking error signal TES indicates a deviation between a track, not shown, formed on the recording face 92a and the light beam 93 in the radial direction of the optical disk 92.
The focus error signal FES is fed to a focus actuator 99 inside the optical pickup 91 through a phase compensation circuit 96, a switch 97, and a driver 98. Thus, the focus actuator 99 drives the lens 94 in the direction perpendicular to the recording face 92a so as to vary the converging position 93a of the light beam 93. A focus search circuit 90 allows the driver 98 to drive the focus actuator 99 in order to make a focus-searching operation, which will be described later. The controller 101 monitors the focus error signal FES, and conducts management and control of the entire focusing device.
The following description will discuss the relationship between the deviation of the recording face 92a and the converging position 93a, and the focus error signal FES. FIG. 40 is a graph that indicates the deviation .DELTA.x between the recording face 92a and the converging position 93a in its horizontal axis as well as indicating the amplitude of the focus error signal FES in its vertical axis.
The origin in FIG. 40 represents a just-focus state in which the converging position 93a coincides with the recording face 92a. The position of the lens 94 corresponding to this just-focus state is hereinafter referred to "just-focus position". The left side of the origin represents a FAR area where the lens 94 is located far from the recording face 92a in relation to the converging position 93a, and the right side of the origin represents a NEAR area where the lens 94 is located near the recording face 92 in relation to the converging position 93a. This drawing shows that the focus error signal FES is a signal which varies in its polarity and amplitude depending on the distance between the recording face 92a and the converging position 93a.
Therefore, in the focusing control device of FIG. 39, if the focus-error generation circuit 95 is designed so as to release the focus error signal FES having the polarity as shown in FIG. 40 and if it is also designed so as to allow the focus actuator 99 to drive the lens 94 in a far direction from the optical disk 92 upon receiving a plus signal in its phase compensation circuit 96, it is possible to provide control so that the converging position 93a is always focused onto the recording face 92a.
However, it is only within a range of several tens of .mu.m in either the near or far direction with respect to the just-focus position that the above-mentioned effective focus error signal FES is obtained. In other words, in an out-focus state beyond this range, the focus error signal FES becomes virtually zero irrespective of the converging position 93a as shown in FIG. 40. For this reason, the positional relationship between the converging position 93a and the recording face 92a becomes rather indefinite, making it difficult to provide effective focusing control. Consequently, in order to provide an effective focusing servo operation, it is necessary to keep the converging position 93a within several tens of .mu.m from the just-focus position.
However, in most cases, when a focusing servo operation is started, the converging position 93a stays out of this range. For this reason, a focus search operation is normally conducted before the focusing servo operation in order to search for a range in which the effective focus error signal FES is obtained.
In FIG. 39, the controller 101 switches the switch 97 on the terminal-E side so that the output of the focus search circuit 90 is supplied to the driver 98. Thus, the driver 98 drives the focus actuator 99 so that the lens 94 is moved in the direction perpendicular to the recording face 92a to a great degree. In this case, if the output of the focus search circuit 90 is given as a wave that varies with time, such as a triangular wave or a sine wave, the lens 94 is moved in the approaching direction or in the departing direction with respect to the recording face 92a; this causes the focus error signal FES to vary as is shown in FIG. 41.
A zero-cross point, which appears in the middle of the course during which the focus error signal FES varies from the plus peak (or the minus peak) to the minus peak (or the plus peak) having the opposite polarity, represents the just-focus state, which has been described in FIG. 40. Therefore, the controller 101 switches the switch 97 onto the terminal-D side so as to start the focusing servo operation at the zero-cross point of the focus error signal FES or in the vicinity thereof. A sequence of processes in which the focus search operation is switched to the focusing servo operation is generally called a focus pull-in operation.
Here, the total of a tolerance due the warp of the optical disk 92 and a mechanical tolerance of the optical pickup 91 or the focus actuator 99 is estimated to be several hundreds of .mu.m. For this reason, the signal to be released by the focus search circuit 90 is formed so as to have an amplitude that is large enough to allow the focus actuator 99 (and the lens 94) to move at least not less than this total tolerance. Thus, it becomes possible to always obtain an effective focus error signal FES during the focus search operation.
In order to shorten the time required for the focus pull-in operation, the following two methods are employed. One method is to increase the frequency of the output signal from the focus search circuit 90 in order to increase the frequency of passages per unit time of the focus error signal FES with respect to the zero-cross point, that is, a target for the focus pull-in operation.
However, this method raises the following problems.
As described earlier, the range in which the focus pull-in operation is available is restricted to several tens of .mu.m before and after the just-focus position. Therefore, in comparison with the movement having a range of several hundreds .mu.m during the focus search operation, the above-mentioned range is very small, only reaching several percent to 10 or 20 percent of the range. For this reason, when movable parts such as the lens 94 receive great momentums from the focus actuator 99 during the focus search operation, it is impossible to stop the movement of the lens 94 within the range of several tens of .mu.m before and after the just-focus position, even if the controller 101 switches the switch 97 onto the terminal-E side at to the zero-cross point of the focus error signal FES. As a result, the lens 94 tends over shoot and to move up to an out-focus position. Therefore, the positional relation between the converging position 93a and the recording face 92a becomes indefinite, resulting in a failure in the focus pull-in operation.
The higher the frequency and amplitude of the signal released by the focus search circuit 90 become, the higher the possibility of failure in the focus pull-in operation. Moreover, a phenomenon wherein the recording face 92a of the optical disk 92 runs out in the perpendicular direction due to the rotation of the optical disk 92, that is, so-called facial vibration, tends to increase the relative velocity between the recording face 92a and the lens 94; this further increases the possibility of failure in the focus pull-in operation.
For example, in Japanese Laid-Open Patent Publication No. 220230/1990 (Tokukaihei 2-220230), there is an example of such techniques that are adopted to improve the focus pull-in operation in order to solve the above-mentioned problem. In this technique, consideration is given to the way of changes in a signal (sum signal) indicating the quantity of reflected light from the optical disk 92 and the focus error signal FES that are obtained in the vicinity of the just-focus position at which the effective focus error signal FES is obtained. More specifically, in the case where the levels of ac components of the focus error signal FES and the sum signal exceed predetermined values, that is, in the case where the focus actuator 99 is about to move the lens 94 apart from the just-focus position, a signal having a polarity to allow the focus actuator 99 (and the lens 94) to approach the just-focus position is supplied to the focus servo system instead of the focus error signal FES. With this arrangement, the focus error signal FES is shaped so that the range enabling the focus pull-in operation can be expanded.
In the technique disclosed in this patent publication, in order to make a decision as to whether the lens 94 is located in the proximity of the just-focus position or at an out-focus position, circuits for extracting ac components of a plurality of signals used for generating the sum signal and the focus error signal FES and circuits for adjusting the phases of the ac components are combined together and utilized. Further, in order to expand the range enabling the focus pull-in operation, the focus error signal FES is shaped while adjusting the levels and timings of these signals in a very sensitive manner.
However, the problem of the arrangement of the above-mentioned patent publication is that a complicated circuit configuration is necessary because many circuits including comparators, high-pass filters, low-pass filters sample-hold circuits, etc. are employed.
Further, when the optical disk 92 is replaced, the level of the sum signal in the proximity of the just-focus position, the level of the focus error signal FES, and the degree and velocity of the facial vibration also change; this raises another problem that desirable performance is not obtained without readjusting the time constants of the high-pass filter and low-pass filter as well as readjusting the reference level of the comparator.